The present invention is directed to devices and methods for simultaneously oxygenating, and heating or cooling the blood during surgery. In particular, the present invention is directed to a device which includes an integrally formed heat exchange element and membrane oxygenator.
Devices which heat or cool, and oxygenate the blood are typically used in surgical operations where the supply of blood from the heart is interrupted, e.g. during open heart surgery, or any other surgery performed on the heart or lungs. These devices have been constructed as separate units which are interconnected in an extracorporeal circuit, or have been constructed as a single unit which incorporates the two different devices in separate compartments.
An oxygenation device would be either one of two different types of oxygenators, a bubble oxygenator or a membrane oxygenator. Bubble oxygenators include a gas sparger into which a stream of oxygen bearing gas is directed for forming and dispersing gas bubbles in the blood to perform the oxygenation. Membrane oxygenators direct the blood into contact with a surface or membrane through which gas can diffuse or be transferred. These surfaces or membranes are used to transfer oxygen and carbon dioxide between the blood and an oxygen bearing gas. The benefits and disadvantages of both types of oxygenators are well known and will not be dealt with in any detail herein.
A heat exchange device usually includes a hollow structure, e.g. a coil formed from a thermally conductive material, upon which the blood is directed, while a fluid is passed through the coil. The fluid is either heated or cooled to appropriately heat or cool the coil, and thus the blood.
In recent years the use of membrane oxygenators has increased over the use of bubble oxygenators. This preference of membrane oxygenators over bubble oxygenators has developed because of the damage caused to the blood components when using bubble oxygenators. The precise reasons for this increased use of membrane oxygenators will not be discussed in any greater detail herein.
Commercially available membrane oxygenators include tubing or hollow fibers which are formed from a material through which gas may diffuse under the proper operating conditions. The blood is either passed through the tubing or fibers, with an oxygen bearing gas passed about the fibers, or conversely the blood can be passed about the tubing or fibers with the oxygen bearing gas passed there through. The tubing or the hollow fibers may be formed from silicon, e.g. silicon tubing, or be formed as a porous fiber from a hydrophobic polymeric material.
While some commercially available bubble oxygenators have been provided in a single compartment with a heat exchange device, no commercially available membrane oxygenator has been incorporated with a heat exchange device in a single compartment. That is, the heat exchange device and membrane oxygenator are either provided in separate housings, or the heat exchange and oxygenator are provided in two separate compartments of a single housing. Specifically, the individual compartments may be formed in two separate housings which are joined together to form a single entity, or a single housing is molded with two separate compartments. The blood is transferred between the individual compartments through either tubing, which is connected therebetween, or a manifold which is formed with a fluid passageway connecting one compartment to the other. In either event, both of the compartments will have to be filled with blood in order to adequately perform the heat exchange and the oxygenation.
Examples of various types of units including a heat exchange device and a membrane oxygenator arranged in separate housing compartments joined together to form a single unit are disclosed in U.S. Pat. Nos. 4,261,951, issued to Milev on Apr. 14, 1981; 4,376,095, issued to Hasegawa on Mar. 8, 1983; 4,424,190, issued to Mather, III et al, on Jan. 3, 1984; and 4,657,743, issued to Kanno on Apr. 14, 1987, and European Patent Application Number 176,651, filed by Mitsubishi Rayon Co. Ltd, on Feb. 14, 1985.
The primary disadvantage with commercially available membrane oxygenators and heat exchange units pertains to the relatively large priming volume of the extracorporeal circuit which includes such devices. Prior to the initiation of surgery the total internal volume of the extracorporeal circuit, which includes the oxygenation and heat exchange devices, as well as other devices must be primed. Priming is performed to flush out any extraneous gas from the extracorporeal circuit prior to the introduction of the blood, and is typically performed with any biocompatible solution, e.g. a saline solution.
The larger the priming volume the greater the amount of priming solution present in the circuit which mixes with the patient's blood. The mixing of the blood and priming solution causes hemodilution, that is the dilution of the blood cells, and in particular the red blood cells for a given volume of fluid. Hemodilution can be disadvantageous since the concentration of blood cells, e.g. red blood cells, must be maintained during the operation in order to minimize adverse effects to the patient. In order to reduce the disadvantage of hemodilution, donor blood, that is blood from other than the patient, is introduced into the diluted blood passing through the extracorporeal circuit. The addition of this blood is performed to raise the blood cell count.
While the addition of this donor blood reduces the disadvantages associated with hemodilution, the donor blood presents other complications, such as compatibility problems between the donor blood and patient's blood, and complications associated with blood borne diseases.
Additional measures taken to correct the effects of hemodilution include the use of hemoconcentrators. Hemoconcentrators are used to concentrate the blood cell count in a given volume of blood. These types of devices are connected in the extracorporeal circuit, and remove a portion of the fluid of the blood which is concentrated by the reduction of fluid. Such devices are expensive and cumbersome to operate.
Thus the increased priming volume created by the use of membrane oxygenators and heat exchange devices in separate compartments or housings presents the disadvantages of hemodilution, and also those disadvantages associated with the correction of the hemodilution.
Another disadvantage with a large priming volume is the amount of time expended in priming the circuit, which increases the start-up time for the surgery. A still further disadvantage with the use of membrane oxygenators and heat exchange devices in separate housings or compartments, is the necessity of connecting such compartments with tubing, or with a manifold passageway. The need to interconnect such compartments or housings with surgical tubing further increases the assembly time of the extracorporeal circuit.
It is thus apparent that it would be advantageous to design a device incorporating a membrane oxygenator and heat exchange device into a single housing compartment. This type of device would reduce the priming volume, and the associated disadvantages with a larger priming volume, and would also reduce the start-up time. Another advantage by combining the heat exchange device and the membrane oxygenator in a single compartment is the reduction in manufacturing cost due to the reduced amount of materials, parts and labor to construct a single housing compartment, instead of two compartments, or two separate housings.
The major impediment in providing the heat exchange device and oxygenator in a single compartment is the requirement of providing sufficient surface area for both heat exchange and for oxygenation. That is, any such combination would have to be able to provide for sufficient surface area to allow for an adequate exchange of heat, and also to perform the desired oxygenation.
A suggested approach to the incorporation of the heat exchange device and the oxygenator in a single compartment is taught in U.S. Pat. No. 4,306,018, issued to Kirkpatrick on Dec. 15, 1981. The gas-heat exchange device illustrated includes a central heat exchange core formed from a thermally transmissive material mounted in a housing. A heat exchange fluid is passed through this core to regulate the surface temperature of the core. The blood is passed through a silicone tube which is wrapped about the exterior surface of the core. The transfer of heat is performed across the core surface and silicone tubing interface. Oxygenation of the blood traveling through the silicone tubing is performed by passing an oxygen bearing gas about the tubing wrapped about the core.
This type of device is disadvantageous because of the inadequacy of the heat transfer, since the heat transfer must occur across the silicone metal interface. Even if the blood is passed outside the silicone tubing, with the gas passed there through, the amount of tubing necessary to effect an adequate gas transfer would require either a substantial wrapping about the core, thus impeding the contact between the blood and the core, or require a relatively large structure to provide for the necessary surface area to obtain adequate heat exchange and oxygenation.
Another device is taught in U.S. Pat. No. 4,715,953, issued to Leonard on Dec. 29, 1987. The device taught in this patent is only described as useful for dialysis, and is not taught, nor intended to perform both gas and heat exchange. The described device includes a plurality of hollow fiber membranes wrapped entirely over the surface of a central core. That is, since the purpose of the device is to provide for an adequate gas exchange the surface of the core is completely covered by the hollow fibers. This eliminates the exposure of the core surface, and thus the ability to use the central core as a heat exchange element. Furthermore, even if the device was modified to pass the blood through the hollow fibers, as is done in the Kirkpatrick patent discussed above, the heat transfer would have to occur across the membrane-metal interface. The resulting device would thus suffer the same disadvantages of the Kirkpatrick device as discussed above.
There thus remains a need to provide a device which would include a membrane oxygenator and heat exchange device as a single integral unit without the disadvantages discussed above.